The invention is generally directed to an optical head for an optical memory device utilizing an optical disk and in particular to a driving mechanism for an objective lens actuator which forms the front portion of an optical head.
Reference is made to FIGS. 1 and 2 wherein an example of an objective lens actuator constructed in accordance with the prior art is depicted. The actuator includes a shaft 1 fixed on a magnetic yoke 2. An annular cylindrical member 3 is formed on yoke 2 around shaft 1. Element 4 is a magnet arranged atop the perimeter of yoke 2. Annular yoke 5 is mounted on top of magnet 4. A bobbin 6 made of a non-magnetic material is sized so as to slide vertically along shaft 1 and to rotate around shaft 1. Bobbin 6 has a focusing coil 7 wound about its outside surface in the manner of a standard speaker coil. In addition, a tracking coil 8 is wound about bobbin 6 and focusing coil 7 in a frame-like form as seen clearly in FIG. 2. Tracking coil 8 is mounted mechanically to bobbin 6 in one piece with focusing coil 7. Tracking coil 8 is provided with four components distributed circumferentially about bobbin 6.
Annular yoke 5 has two cutaway portions 5a so that only one of the two vertical legs of each of the four frame-like coils 8 is positioned between annular yoke 5 and bobbin 6. This produces an electro-magnetic effect which results from only one of each of the two vertical legs of the four frame-like components of tracking coil 8. The transmitted light path 3a, 6a of the optical beam moves through objective lens 9. Contact wires 7a and 8a are the contacts for introducing current into focusing coil 7 and tracking coil 8, respectively.
In accordance with the above-described structure a magnetic circuit .phi. is formed by magnet 4 (FIG. 1). As a result when current flows through focusing coil 7, focusing coil 7 and bobbin 6 move upward or downward depending upon the direction of the current. The distance of movement of bobbin 6 is dependent upon the magnitude of current present in focusing coil 7. As a result, the laser light passing through objective lens 9 is accurately focused on the optical disk (not shown). The optical disk would be above objective lens 9.
When a current is applied to tracking coil 8 a force to rotate bobbin 6 around shaft 1 is produced, the direction of rotation depending upon the direction of current through tracking coil 8. The rotation of bobbin 6 causes objective lens 9, which is near the perimeter of bobbin 6, to rotate about an arc, where the radius is the distance between shaft 1 and objective lens 9. As a result, over small arcs of rotation the movement of objective lens 9 is approximately linear.
Objective lens actuators constructed in accordance with the prior art utilize objective lenses which rotate about an axis in the center of the actuator. As a result, the diameter of the actuator unit as a whole inevitably becomes large to minimize the non-linearity of movement of the objective lens. Consequently, it is difficult to reduce the size of the optical head.
This conventional objective lens actuator is not suitable for a small-sized optical head. If the size of the optical head is forced to be scaled down to a smaller size, the radius of the arc of rotation through which the lens rotates is also reduced. As a result, the range in which the movement of the lens in the tracking direction can be regarded as linear is limited and, outside of this range, the variation of the optical axis of the light reflected by the optical disk increases. As a result, accurate servo-control is not realized. This results in a reduction in the range in which the optical head can follow fluctuations in the track. Accordingly, there is a need for a small, thin actuator in which the optical head tracking range is wide without non-linearities or increases in the variation of the optical axis of the light reflected by the optical disk.